I'm going through the same problems. I already changed two 16gb miniSD cards. Then I thought about using an 8gb mini SD card but after I transferred some files into the card, it started rebooting every 5 mins. Really frustrating! I have a Samsung Galaxy S2 from T-Mobile and have only used it for about 1 month.
Originally I thought the miniSD cards are defective so I went back to BestBuy to exchange twice, then I got a smaller 8GB but it's the same problem. If I take out the miniSD card, then the phone goes back to normal, no rebooting at all. I'm not sure if the memory card slot is defective on my phone?